1. Technical Field
This disclosure relates to synchronizing video presentations, and more particularly, to systems that synchronize video presentations to a master time reference through a modified video cadence.
2. Related Art
When presenting video content having a frame rate that differs from the frame rate of a targeted output device, a “cadence” may maintain a smooth visual display. To display a 24 frame-per-second (fps) movie on a National Television System(s) Committee (NTSC) format at 30 fps, a 3:2 cadence can be used. In a 3:2 cadence pattern, an input frame is displayed for 3 output frame durations before the next input frame is displayed for 2 output frame durations. The cadence pattern is then continuously repeated. Other cadence patterns can be used, for example, for 25 fps and 30 fps source video, and 50 fps Phase Alternating Line (PAL) display frame rates.
It may be desirable to synchronize a video presentation to an audio presentation. In one approach, the timing of the audio presentation may serve as a master time reference and the timing of presentation of the video frames is regulated to coincide with the master time. In this approach a correct cadence pattern may not be maintained as the audio clock may not be in synchronization (a.k.a. in sync) with the frame boundaries of the video display (a.k.a. vertical sync). The audio clock and the vertical sync may be subject to drift over time that results in a visual judder. In a second approach, the vertical sync interval of the video display is used as the master time reference and the audio clock is adjusted to coincide with the vertical sync intervals. In this approach, the individual frames of the audio stream may need to be stretched or compressed, requiring relatively complex computing-intensive processes to maintain synchronization. Such processes may render audio artifacts that may be perceived by a listener.